CBSE Questions for Class 12 Medical Biology Principles Of Inheritance And Variation Quiz 2 - MCQExams.com

Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. This method has long been used in horticulture, e.g. to incorporate desirable traits to the elite line of crops. At present, the deliberate back crossing is carried out to breed and produce gene knockouts used in Genetics research.

One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented by an English Geneticist named as Reginald Punnett. His technique was thus popularly accepted as Punnett square. It is the simplest graphical way of discovering all possible combinations of genotypes that may occur in offsprings.

• Seven characters found in garden pea plants were selected by Mendel for his experiment. 
• They were seed shape, seed colour, flower colour, pod shape, pod colour, flower position, and stem height. 
• For the experiments, Mendel selected a set of two contrasting traits for each of these characters. 

• The hybrid breakdown is a reduction in fitness as seen in the grandchildren of the two individuals from separate gene pools that participated in the hybridization event to begin with. 
• Hybrid breakdown, in other words, is something that happens to the offspring of hybrids rather than to hybrids themselves.
• As a result of hybrid breakdown, the F$$_2$$ generation will become inferior to F$$_1$$ generation.

A monohybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in a single given trait. 

Trait: Flower colour

Dominant trait: Red colour

Recessive trait: White colour

The above cross represents two homozygous individuals, one with the dominant trait (red flowering plant, RR) and the other with the  contrasting recessive trait (while flowering plant, rr). The F$$_{1}$$ generation of these resulted in all heterozygous red flowering individuals (Rr). The F$$_{1}$$ generation is self crossed to produce the F$$_{2}$$ generation and the results are homozygous and heterozygous red flowering plants (RR, Rr) and homozygous white flowering plants (rr). It is important to point out here that Mendel's experiment and his resultant statistical analysis showed that the total offspring of the F$$_{2}$$ generation was 75% red flowering plants and 25% white flowering plants or a 3:1 ratio. If two homozygous traits are crossed, the phenotype of the F$$_{1}$$ is called the dominant trait. When two F$$_{1}$$ plants are crossed, the F$$_{2}$$ phenotype will have representatives of the dominant trait and the recessive trait (the recessive trait will remain hidden in the F$$_{1}$$ and reappears in the F$$_{2}$$). The phenotypic ratio in the F$$_{2}$$ will be 3:2:1, dominant to recessive. Therefore, the F$$_{2}$$ generation of a monohybrid cross, with dominance,  will result in Phenotypic ratio of 3:1 and Genotypic ratio of 1:2:l.

• Pedigrees are charts of family histories that show the phenotypes and family relationships of the individuals using symbols to represent different family members. 
• Squares represent males, circles represent females, filled-in-symbols represent individuals who have a genetic condition, and a probandis the first affected family member that seeks medical attention. 
• Such pedigree charts are used for human genetic counseling.

• The pedigree is showing that in generation 1, the male is affected and the female is unaffected.
• In generation 2, the females are affected and the males are unaffected. This means the female in generation 1 was a carrier for a particular trait.
• In generation 3, both males and females are affected. 

• The pedigree chart shows the inheritance of an autosomal recessive trait, as children of normal parents are affected. This is possible only in the case of a recessive inheritance because both parents are carriers of the disease.
• Apart from that, both male child and female child are affected in second-generation so there are higher chances that pedigree belongs to an autosomal inheritance.
• An example of an autosomal recessive trait is phenylketonuria.